1. Field of the Invention
The present invention relates to a jet or flow soldering method and apparatus for soldering electronic circuit parts onto a printed circuit board.
2. Description of Related Art
In a jet (flow) soldering method, as shown in FIG. 18, a printed circuit board (substrate) 5 is transferred by a conveyer 94 to stick solder onto a bottom surface (treatment surface) of the substrate 5. Specifically, the substrate 5 is transferred over a pre-heater 7, a primary jet (flow) soldering bath 2 and 20 a secondary jet (flow) soldering bath 3 sequentially at a fixed speed by the conveyer 94.
Thus, the substrate 5 is heated at above the pre-heater 7, so that a temperature difference between the top surface and the bottom surface of the substrate 5, which will occur at the 25 time of soldering, is reduced to prevent warping of the substrate 5. Then, the solder stick onto the treatment surface of the substrate 5 to solder electronic circuit parts to the treatment surface by transferring the substrate 5 over the primary jet soldering bath 2 with the treatment surface of the substrate 5 being immersed in a primary solder jet 21, that is, contacted to the primary solder jet 21. Thereafter, the solder sticking onto the substrate 5 is shaped by transferring the substrate 5 over the secondary jet soldering bath 3 with the treatment surface being immersed in a secondary solder jet 31.
Here, as shown in FIG. 19A, the solder stick onto the treatment surface 51 of the substrate 5 by waving the primary solder jet 21 in the primary jet soldering bath 2. In a case that space 57 between electrode terminals of electronic circuit parts 59 on the substrate 5 is comparatively large as shown in FIG. 19A, the solder is enabled to enter and stick to lands 58 by waving the primary solder jet 21. However, the magnitude of wave is limited because of its dependency on the viscosity, the surface tension and the like of solder. As a result, in a case that the space 57 between the electronic circuit parts 59 on the substrate 5 is small as shown in FIG. 19B, the solder is disabled to enter the space. Thus, defect of imperfect soldering increases, as the substrate 5 is integrated highly recently, that is, as the circuit parts 59 are located more closely.
Further, the temperature distribution in the substrate 5 is likely to become non-uniform because of influence of non-uniformity of temperature in the upper surface (heat radiation surface) of the pre-heater 7. This causes warping of the substrate 5, and damage to the electronic circuit parts 59 when heated excessively. The pre-heated temperature varies from substrate to substrate. The pre-heating condition is determined only by the temperature, resulting in an impediment to equalization of the pre-heated temperature among the substrates.